linux-kernel - Re: Profiling execution time for __alloc_pages

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Date:   Tue, 11 May 2021 11:01:36 +0530
From:   Shivank Garg <shivankgarg98@...il.com>
To:     Yang Shi <shy828301@...il.com>,
        David Hildenbrand <david@...hat.com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        gregkh@...uxfoundation.org, sergey.senozhatsky@...il.com,
        pmladek@...e.com
Subject: Re: Profiling execution time for __alloc_pages_nodemask

>Are you maybe calling ktime_get() too early during boot, where some
>subsystems are just about to be brought up?

Thanks David, I guess this is the issue.

>Is there any reason that prevents you from using some advanced tools,
>i.e. perf, bcc/bpftrace, etc? They are much simpler than adding
>instrumentation in kernel.

Actually, I want to record the arguments for the memory allocator and
filter out the calls I don't need (which are creating noise).
For instance, I'm only interested in particular order and flags.

This is the reason I added my custom tracepoint, which theoretically
looked easy to implement without adding much overhead.

Thanks,
Shivank

On Tue, May 11, 2021 at 5:30 AM Yang Shi <shy828301@...il.com> wrote:
>
> On Mon, May 10, 2021 at 7:57 AM Shivank Garg <shivankgarg98@...il.com> wrote:
> >
> > Hi Everyone!
> >
> > I want to profile the time taken to execute the __alloc_pages_nodemask
> > for different linux configurations/parameters.
> > To measure the execution time, I use the ktime_get() apis. I get the
> > ktime_get() on the top, and I want to do ktime_sub(ktime_get(),ktime)
> > and record it in a tracepoint.
> > However, the patch on implementation prevents the kernel from booting
> > up. I debugged the bug to find out that the issue recurs on adding
> > ktime_get() inside the __alloc_pages_nodemask path. So, that the
> > kernel fails to boot up ( and show the blank screen without any logs )
> > I'm using the Linux kernel 5.6.13 (5821a5593fa9f28eb6fcc95c35d00454d9bb8624)
>
> Is there any reason that prevents you from using some advanced tools,
> i.e. perf, bcc/bpftrace, etc? They are much simpler than adding
> instrumentation in kernel.
>
> >
> > Is it an expected behavior? or a BUG? Has anyone else faced the same issue?
> >
> > Can you please suggest, what would be a good way to measure execution
> > time for page allocation (if not ktime_get)
> >
> > Stay Safe!
> > Best Regards,
> > Shivank Garg
> > Open-Source Enthusiast and Student, IIT Kanpur
> >